1. Field of the Invention
The present invention generally relates to an electrically powered steering device for an automotive vehicle and, more particularly, to the electrically powered steering device employing a ball screw mechanism for transmitting a drive output from an electric motor to a steering shaft to selectively advance and retract the latter.
2. Description of the Prior Art
The electrically powered steering device is an instrument to assist the steering force of a steering wheel by means of an electrically driven motor and is currently available in various types. One of the types employed is of a design wherein a retractable steering shaft coupled with a steering mechanism for vehicle wheels is imparted an axially shifting force that is transmitted thereto from the steering wheel through a motion translating mechanism such as a rack-and-pinion mechanism for translating a rotary motion of the steering wheel into the axially shifting motion and, also, an axially shifting force that is transmitted thereto from an output of the electrically driven motor through a ball screw mechanism. The ball screw mechanism is currently available in various types according to the manner in which a series of balls are circulated, including a bridge type, a tube circulating type and an end-cap type. The electrically powered steering devices of a rack-and-pinion type employing these ball screw mechanisms are disclosed in, for example, the Japanese Laid-open Patent Publication No. 59-50864 disclosing a basic structure, the Japanese Laid-open Patent Publication No. 9-142315 disclosing the use of the bridge type ball screw mechanism, the Japanese Laid-open Patent Publication No. 54-47236 disclosing the tube (circulator) type ball screw mechanism, and the Japanese Laid-open Patent Publication No. 6-20 1013 disclosing the use of a resin-made end cap type and a resin-made circulating pipe type.
FIG. 23A illustrates the prior art ball screw mechanism of the bridge type such as disclosed in the Japanese Laid-open Patent Publication No. 9-1423 15. As shown therein, a ball screw shaft 1 having an externally threaded groove 4 formed thereon has a ball screw nut 5 mounted therearound. The ball screw nut 5 has an inner peripheral surface formed with an internally threaded groove 5b in alignment with the externally threaded groove 4 to thereby define a ball guide passage for a series of balls 7. The ball screw nut 5 is arranged with a bridge member 5d that defines a connecting passage for connecting neighboring internally threaded grooves 5b. When the ball screw nut 5 rotates about and relative to the ball screw shaft 1, the ball screw shaft 1 is axially moved relative to the ball screw nut 5 by the effect of a reactive force from the threaded grooves. Since the balls 7 successively depart from the ball guide passage as they rollingly advance along and within the threaded grooves incident to rotation of the ball screw nut 5 and axial movement of the ball screw shaft 1, a supplement of the balls would be required depending on the amount of rotation. In view of this, the provision has been made of the bridge member 5b so that the balls 7 successively advanced along and within the threaded grooves can be returned to the threaded grooves through the connecting passage defined in the bridge member 5d. 
On the other hand, shown in FIGS. 23A and 23B is an example of the prior art bridge type ball screw mechanisms that is disclosed in the Japanese Laid-open Patent Publication No. 54-47236. As shown in FIGS. 23A and 23B, of the groups of balls lined up in a region LA or LB in an axial direction of the ball screw nut 2, that is, the balls of either one of two rows of the balls rollingly move within threaded grooves formed in the ball screw shaft 1 over a distance corresponding to an axial length and interposed between the ball screw grooves. The two rows of balls 3A and 3B guided by respective rows of circulators 6A and 6B so as to circulate are interposed between the internally threaded groove 2a in the ball screw nut 2 and externally threaded grooves 1a and 1b in the ball screw shaft 1 while the balls of one row 3A are so designed as to have a diameter appropriately larger than that of the balls of the other row 3B, so that during the straight run of the automotive vehicle the balls of the row 3A can roll in part within the internally threaded groove 2a in the ball screw nut 2 and in part within the internally threaded groove 1a in the ball screw shaft 1 which has a large diameter portion d2 while the ball of the row 3B can roll in part within the internally threaded groove 2a in the ball screw nut 2 and in part within the externally threaded groove 1b in the ball screw shaft 1 which have a reduced diameter portion d1. Accordingly, the row of the balls 3A of the larger diameter that are guided by the circulator 6A are interposed between the internally threaded groove 2a in the ball nut 2 and the externally threaded groove 1a in the large diameter portion d2 of the ball screw shaft 1 with no gap formed therebetween. In this design, when the steering wheel is turned rightwards (clockwise) or leftwards (counterclockwise) during the steering of the automotive vehicle, accompanied by rotation of the ball screw shaft 1, the ball screw nut 2 can be moved rightwards or leftwards through the threaded grooved and the rows of the balls 3A and 3B.
One example of the prior art end cap type ball screw mechanism is disclosed in the Japanese Laid-open Patent Publication No. 6-201013. According to this patent publication, an end cap is mounted on each of opposite ends of the ball screw nut to allow the balls to be returned to the circulating pipe and the end caps and the circulating pipe are both made of a synthetic resin.
FIG. 24 illustrates another type of the prior art end cap type ball screw mechanism. As shown therein, a rotary nut 71 has a substantially intermediate portion thereof formed with a radially outwardly protruding flange 72. A rolling bearing 73 is mounted externally on the rotary nut 71 so that the rotary nut 71 can be rotatably supported by and within a housing. A rotor 74 of an electrically driven motor for driving the rotary nut 71 about the longitudinal axis thereof is also mounted on the rotary nut 71. The radially outwardly protruding flange 72 is utilized for positioning the rolling bearing 73 and the rotor 74 relative to the rotary nut 71.
The rolling bearing 73 is of a so-called inner-race rotating type in which the inner race 73a is rotatable together with the rotary nut 71 and, for this purpose, the rotary nut 71 is press-fitted into the inner race 73a with a predetermined interference present between an inner peripheral surface of the inner race 73a and an outer peripheral surface of the rotary nut 71. On the other hand, since the rotor 74 cannot be press-fitted over the rotary nut 71 in a manner similar to the rolling bearing, a portion of the outer peripheral surface of the rotary nut 71 that is connected with the rotor 74 is knurled to provide a knurled surface area 75 for slipless engagement with the rotor 74. While the knurled surface area 75 can be formed by any known rolling process, an annular groove 76 is necessarily formed in the rotary nut 71 on each side of the knurled surface area 75 for the convenience of the rolling process. Also, a portion of the outer peripheral surface of the rotary nut 71 on one side of the knurled surface area 75 adjacent the end thereof is formed as a cylindrical guide portion 77 for guiding the rotor 74 onto the knurled surface area 75 before the rotor 74 is mounted on the knurled surface area 75 in a slipless fashion. The cylindrical guide portion 77 has a diameter slightly smaller than the outer diameter of the knurled surface area 75 of the rotary nut 71 so as to facilitate mounting of the rotor 74 onto the rotary nut 71.
Although the prior art bridge type shown in FIG. 23A has an advantage in that the ball screw nut 5 can have a relatively small outer diameter, not only is the number of component parts large because of the use of the separate bridge member 5d mounted on an outer diametric portion of the nut 5 that is depleted, but also difficulty is encountered in mounting a motor and a bearing onto the outer peripheral surface of the ball screw nut because of the use of the separate bridge member 5d mounted externally on the nut as a ball circulating component part. In addition, only chamfering can be effected to that end of the bridge member 5d secured to the nut 5 at an inner periphery of the ball screw nut 5 and, therefore, a step tends to be necessarily occur at the joint between the bridge member 5d and the ball screw nut 5. The step so formed will scrape a film of the lubricant deposited on the surface of each ball 7.
Also, since the bridge member 5d requires to be firmly connected in position with a bonding material filled in a depleted portion of the outer diameter of the ball screw nut 5, not only is the workability lowered, but also the bonding material used tends to come oozing, requiring a job of removing the deposited bonding material.
Where in the bridge type is applied to the ball screw in which the outer diameter of the nut is limited, the following limitation tends to occur. Specifically, in order to secure a high load capacity with the outer diameter of the nut reduced and having a reduced sectional height, it is necessary to accommodate an increased number of balls of a small size. Although in such case the employment of multi-thread grooves capable of increasing the load capacity is advantageous, the employment of the multi-thread grooves requires the bridge member 5d for returning the balls to be disposed so as to straddle the adjacent ball guide passage and, therefore, the employment of the multi-thread grooves is impractical and impossible.
In the prior art bridge type shown in FIGS. 23B and 23C, where the balls movable so as to circulate within the ball screw have the same diameters, the outer diameter of the ball screw nut 2 tends to increase, resulting in increase of the entire size of the steering device used in, for example, an automotive vehicle. Also, where a rack or the like is provided on the outer periphery of the nut, areas to be machines are limited and, therefore, the phase of the ball screw aligned with a neutral position of the steering wheel of the electrically powered steering device must be supervised.
Although the prior art bridge type shown in FIG. 23A has an advantage in that the screw nut 5 can have a relatively small outer diameter, not only is the number of component parts large because of the use of separate bridge member 5d mounted on an outer diametric portion of the nut 5 that is depleted, but also difficulty is encountered in mounting a motor and a bearing onto the outer peripheral surface of the ball screw nut because of the use of the separate bridge member 5d mounted externally on the nut as a ball circulating component part. In addition, only chamfering can be effected to that end of the bridge member 5d secured to the nut 5 at an inner periphery of the ball screw nut 5, and therefore, a step tends to necessarily occur at the joint between the bridge member 5d and the ball screw nut 5. The step so formed will scrape a film of the lubricant deposited on the surface of each ball.
With respect to the prior art end cap type shown in FIG. 14, an outer peripheral surface 71a of the rotary nut 71 adjacent a portion where the bearing is mounted is formed in a cylindrical shape. Accordingly, when the inner race 73a of the bearing 73 is press-fitted onto that portion of the outer peripheral surface 71a of the rotary nut 71, the inner race 73a tends to collide against an annular end edge of the outer peripheral surface 71a, resulting in reduction in workability during the press-fitting of the bearing 73. Even at the time of assemblage of the rotor 74, mounting of the rotor 74 onto the knurled surface area 75 through the guide area 76, utmost care is required to avoid any possible misalignment and formation of the step, requiring the increased number of manufacturing steps. Also, even where the separate end cap 77 is fastened to the nut body of the rotary nut 71 by the use of the bolt, the outer diameter of the end cap 77 is precautiously chosen to be slightly smaller than that of the guide area 76 to avoid any possible misalignment and formation of the step, but the selection of the outer diameters in this way brings about another problem in that the presence of the step interferes with the inner diameter of the rotor 74 at the time of mounting of the rotor 74. On the other hand, while the knurled surface area 75 is recommended to have a length as large as possible, limitation is imposed due to the machining and assembling problems.
Accordingly, the present invention has been devised to substantially eliminate the above discussed problems inherent in the prior art electrically powered steering device and is intended to provide an improved electrically powered steering device wherein the balls in the ball screw mechanism can roll and circulate smoothly and which is simple in structure and can easily be assembled while accomplishing a size reduction of the electrically powered steering device.
In order to accomplish the above described object, in one aspect of the present invention, there is provided an electrically powered steering device which comprises a housing; a steering shaft drivingly connected with a steering mechanism for steering wheels and extending completely through the housing; a motion translating mechanism for translating a rotary force exerted by a steering wheel into a force required to move the steering shaft in a direction axially of the steering shaft; a ball screw mechanism including a ball screw shaft constituted by a portion of the steering shaft, and a rotary nut; and an electric drive motor mounted at one end on an outer periphery of the rotary nut of the ball screw mechanism. The ball screw mechanism comprises the ball screw shaft having a spiral outer groove formed therearound. The rotary nut is formed with a spiral inner groove confronting to and aligned with the spiral outer groove on the ball screw shaft. A plurality of torque transmitting balls are disposed in a series within a ball rolling guideway defined between the spiral outer groove on the ball screw shaft and the spiral inner groove in the rotary nut and for transmitting a force between the rotary nut and the steering shaft. A ball circulating passage communicated with the ball rolling guideway is formed in part in a nut body of the rotary nut and in part in end caps secured to respective opposite ends of the nut body.
In the practice of the present invention, the ball circulating passage may include a circulating tunnel defined in the nut body so as to extend axially thereof, and a passage defined in one or both of an end face of the nut body and an inner end face of the end cap held in contact with such end face of the nut body so as to straddle therebetween for communicating a corresponding end of the circulating tunnel with the ball rolling guideway.
According to the structure described above, since the ball circulating mechanism does not protrude outwardly from the outer periphery of the rotary nut, the rotary nut of the ball screw mechanism can have a reduced outer diameter and the circulation of the balls can take place smoothly. In addition, not only can the number of component parts required be reduced, but also the number of manufacturing steps can be reduced, resulting in reduction in cost of manufacture.
Also in the practice of the present invention, a rolling bearing may be mounted on an outer periphery of the rotary nut for rotatably supporting the rotary nut relative to the housing, in which case the outer periphery of the rotary nut may be formed integrally with a radially outwardly protruding positioning flange for positioning the rolling bearing mounted thereon. This feature permits the rolling bearing for rotatably supporting the rotary nut can be positioned and fixed on the outer periphery of the rotary nut and, therefore, the electrically powered steering device can have a reduced radial size.
Preferably, each of the end caps may have a counterbore that is staked to prevent a fixing bolt, used to secure the respective end cap fixedly to the nut body, from being rotated arbitrarily. According to this feature, after fixing bolts for fixing the end caps to the nut body have been threadingly fastened, the counterbores are staked to lock the end caps relative to the nut body. Accordingly, any possible loosening of the fixing bolts once firmly fastened can be avoided with a simplified structure to thereby increase the reliability.
The ball rolling guideway of the ball screw mechanism may be of a multi-thread design. This feature makes it possible to reduce the diameter of the torque transmitting balls and then to arrange the torque transmitting balls and, therefore, the load bearing capacity can be increased while the radial dimension is reduced to render the device to be compact in size.
An outer periphery of one of opposite ends of a nut body forming a part of the rotary nut and having the spiral inner groove confronting to and aligned with the spiral outer groove around the ball screw shaft, and a circulating tunnel for the balls may be formed as a tapered surface tapering axially inwardly.
This design is particularly advantageous in that at the time the rolling bearing for the support of the nut is press-fitted to the rotary nut and the rotor of the electric drive motor is assembled relative to the rotary nut, the press-fitting of the rolling bearing can be facilitated accompanied by increase in workability because the outer periphery of the end of the nut body is tapered in shape. Also, even a guide during assemblage of the rotor can be performed smoothly because the outer periphery of the end of the nut body is tapered in shape.
Also, a portion of an outer peripheral surface of the nut body that is continued from the tapered surface may be formed into a cylindrical surface and wherein a knurled surface region for avoiding an arbitrary rotation is formed on the cylindrical surface and a portion of the tapered surface adjacent the cylindrical surface.
According to this structure, since the knurled surface region is formed on that portion of the tapered surface, the knurled surface region can have an increased effective width sufficient to ensure a rotational locking effect. Also, in the case where the knurled surface region is formed and if it is made of a sintered alloy, the necessity of use an annular groove required to roll-form the knurled surface region can be advantageously dispensed with and the width of the knurled surface region can be increased correspondingly.
In the practice of the present invention, each of the end caps may have an outer peripheral surface that is tapered in a direction away from the nut body, wherefore the workability in performing the press-fitting of the rolling bearing and the assemblage of the rotor can further be increased.
According to another aspect of the present invention, there is provided an electrically powered steering device which comprises a housing; a steering shaft drivingly connected with a steering mechanism for steering wheels and extending completely through the housing; a motion translating mechanism for translating a rotary force exerted by a steering wheel into a force required to move the steering shaft in a direction axially of the steering shaft; a ball screw mechanism including a ball screw shaft constituted by a portion of the steering shaft, and a rotary nut; and an electric drive motor mounted at one end on an outer periphery of the rotary nut of the ball screw mechanism. The rotary nut of the ball screw mechanism used is so structured and so configured as will be described, below.
This rotary nut is of a so-called xe2x80x9cend cap typexe2x80x9d wherein one of the end caps is formed integrally with the nut body. More specifically, the has defined therein a spiral inner groove confronting to and aligned with a spiral outer groove defined on the ball screw shaft, and a ball circulating passage continued with respective opposite ends of the spiral inner groove and includes a nut body and an end cap firmly connected to one of opposite ends of the nut body. The ball circulating passage includes a circulating tunnel defined in the nut body so as to extend axially thereof, and end passages continued from respective opposite ends of the circulating tunnel to the spiral inner groove, and the nut body has the spiral inner groove, the circulating tunnel of the ball circulating passage and one of the end passages, said end cap having the other of the end passages defined therein.
According to the above described structure, since the joint between the nut body of the rotary nut and the end cap in the ball screw mechanism is found only at one location adjacent the corresponding end of the nut body, the possibility in which rolling motion of the balls in the spiral inner groove and circulation of the balls in the ball circulating passage may be hampered in the presence of the joint can be advantageously reduced about 50% of that which would be brought about by the presence of the two joints. For this reason, rolling motion and lubrication of the balls can be performed smoothly with a reduced possibility of the lubricant film on the surfaces of the balls being removed. Accordingly, the ball screw mechanism can have an increased lifetime. Also, since the rotary nut is made up of the nut body and the only end cap, the number of the component parts is reduced, resulting in an excellent assemblility so that automation of the manufacture of the ball screw mechanism can be facilitated. It is to be noted that since one end of the rotary nut is utilized to support the end cap fastened thereto, mounting of the balls can be carried out while the end cap is removed from the nut body.
In the practice of the present invention, the end cap is preferably connected to one of opposite ends of the nut body adjacent the rotor, and a rolling bearing for supporting the rotary nut may be mounted on the other of the opposite ends of the nut body.
Where this design is employed, the end of the rotary nut where the rolling bearing is press-fitted is completely free from the presence of any step which would be formed on the outer peripheral surface thereof during fastening of the end cap thereto since a portion which ought to be an end cap is integrated together with the nut body, thereby facilitating the press-fitting of the rolling bearing.
Where the end cap is connected to the end of the nut body adjacent the rotor, the outer peripheral surface of the end cap may be formed as a tapered surface tapering axially inwardly in a direction away from the nut body.
Formation of the tapered surface on the outer peripheral surface of the end cap can facilitate assembly by mounting the rotor of the electric drive motor.
The electrically powered steering device of the present invention is featured in that an inner race raceway for the rolling bearing of the ball screw mechanism is formed integrally with the outer peripheral surface of the rotary nut.
According to this structure, the use of an inner race of the bearing for the support of the rotary nut can be dispensed with and, instead, the raceway corresponding in function to the inner race is formed directly on the rotary nut. Therefore, not only can the device as a whole have a reduced outer diameter and be compact in size, but also if the outer diameter remains the same as in the prior art, a sectional height of the rotary nut can be increased, to thereby increase the lifetime and also to provide a relatively large freedom of design choice. Also, since the inner race which is a component part dedicated to the bearing is eliminated and the inner race raceway is formed directly on the rotary nut, a step of press-fitting the bearing inner race onto the rotary nut is advantageously eliminated and the number of the component parts required is reduced, thereby bringing about a meritorious effect on the assembly.
In the practice of the present invention, the end cap may be made of a sintered alloy. The use of the sintered alloy as a material for the end cap makes it possible to employ the injection molding process in which, even when the ball rolling passage is of a multi-thread design, a reversing portion in the end can be accurately and inexpensively formed by injecting the material. By choosing a combination of metallic powdery materials to be mixed together, the physical strength can be increased accompanied by a corresponding increase of the lifetime.
Also, in the practice of the present invention, the nut body may be made of a sintered alloy. The use of the sintered alloy as a material for the nut body makes it possible to employ the injection molding process and the sintering process to form the rotary nut accurately and inexpensively on a mass-production basis, with no need to use any machining process such as turning and grinding. In addition, by suitably choosing a combination of the metallic powdery materials to be mixed together, the required physical strength and the durability can be maintained. Also, unlike the rotary nut made of a synthetic resin, the durability which would be reduced as a result of frictional wear is excellent and a little influence of thermal change is brought about on the dimension, and when the end cap is fastened and fixed to the nut body by the use of bolts, no problem is found which would result from an elastic deformation brought about by the fastening pressure and, therefore, the fastening torque can be properly supervised.
According to a third aspect of the present invention, there is provided an electrically powered steering device which comprises a housing; a steering shaft drivingly connected with a steering mechanism for steering wheels and extending completely through the housing; a motion translating mechanism for translating a rotary force exerted by a steering wheel into a force required to move the steering shaft in a direction axially of the steering shaft; a ball screw mechanism including a ball screw shaft constituted by a portion of the steering shaft, and a rotary nut; and an electric drive motor mounted at one end on an outer periphery of the rotary nut of the ball screw mechanism. The rotary nut used in the ball screw mechanism is of the structure which will now be described.
Specifically, the rotary nut of the ball screw mechanism has defined therein a spiral inner groove confronting to and aligned with a spiral outer groove defined on the ball screw shaft, and a return groove connecting neighboring convolutions of the spiral inner groove to thereby define a circumferential circuit for the balls. Also, a portion of the rotary nut where the return groove is formed is formed integrally with a portion where the spiral inner groove and a portion forming the outer peripheral surface. The term xe2x80x9cintegrally formedxe2x80x9d referred to hereinabove is intended to means non-use of separate component parts connected together.
According to the above described construction, since the rotary nut is of a design wherein the neighboring convolutions of the spiral inner groove are connected together through the return groove, as is the case with the prior art bridge type, no ball return passage is found on the outer periphery of the nut and the rotary nut can therefore have a reduced outer diameter. Moreover, since the return groove is integrally formed in the rotary nut, unlike the bridge type, no step which would be formed when the separate member is mounted is found on the outer peripheral surface of the rotary nut and, therefore, the inner race of the bearing for rotatably supporting the rotary nut can be positioned and fixed on the outer peripheral surface thereof and the rotor of the electric drive motor can be easily mounted. For these reasons, the electrically powered steering device as a whole can be assembled in a compact size. Also, since the return groove is formed integrally in the rotary nut, no joint is formed on an inner peripheral surface of the rotary nut and, therefore, not only can the ball circulate smoothly, but also the lubricant film deposited on the surface of each of the balls will not be scraped off, allowing the ball screw mechanism to have an increased lifetime.
In the practice of the present invention, the spiral outer groove of the ball screw mechanism may have a groove face formed with a hardened layer of a generally uniform depth that is formed by means of a high frequency hardening process. According to this structural design, the depth of the hardened layer formed on the groove face of the spiral outer groove of the ball screw mechanism is uniformly kept at the predetermined value along the curvature of the spiral outer groove and, therefore, the rolling lifetime of the spiral outer groove can be secured and, by allowing the hardened layer not to be formed to a depth greater than necessary, the steering shaft having an excellent toughness and an excellent straightness can be obtained.
Preferably, the hardened layer may advantageously have a Rockwell hardness HRC within the range of 55 to 62 and, in addition thereto or separate therefrom the hardened layer may advantageously have an effective hardened layer depth within the range of 0.20 to 1.10 mm.
In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and:
FIG. 1 is a longitudinal side view of a ball screw mechanism, with a portion broken away, for use in association with an electrically powered steering device according to a first preferred embodiment of the present invention;
FIG. 2 is a longitudinal sectional view, on an enlarged scale, of the ball screw mechanism shown in FIG. 1;
FIG. 3 is a fragmentary longitudinal side view of the ball screw mechanism of FIG. 2, showing a rotary nut employed therein;
FIG. 4 is a longitudinal sectional view showing the rotary nut shown in FIG.3;
FIG. 5 is a perspective view of an end cap forming a part of the rotary nut shown in FIG. 3;
FIG. 6 is a side view showing an injection molding machine used to form the rotary nut;
FIG. 7A is a longitudinal sectional view, with a portion cut out, of a portion of a steering shaft showing a spirally grooved guide face in a condition of being high-frequency hardened in accordance with the teachings of the present invention;
FIG. 7B is a longitudinal sectional view, with a portion cut out, of a corresponding portion of the prior art steering shaft showing a spirally grooved guide face in a condition of being high-frequency hardened;
FIG. 8 is a longitudinal side view of a bearing-mounted ball screw mechanism, with a portion broken away, for use in association with an electrically powered steering device according to a second preferred embodiment of the present invention;
FIG. 9 is a longitudinal sectional view of the bearing-mounted ball screw mechanism shown in FIG. 8, showing the ball screw mechanism;
FIG. 10 is a longitudinal sectional view of a rotary nut employed in the bearing-mounted ball screw mechanism shown in FIG. 8;
FIG. 11 is a longitudinal side view of the ball screw mechanism, with a portion broken away, for use in association with an electrically powered steering device according to a third preferred embodiment of the present invention;
FIG. 12 is a fragmentary longitudinal side view of the ball screw mechanism showing the rotary nut mounted thereon;
FIG. 13 is a longitudinal sectional view of the rotary nut employed in the ball screw mechanism according to the third preferred embodiment of the present invention;
FIGS. 14A and 14B are perspective views of the rotary nut shown in FIG. 13 as viewed from different directions, respectively;
FIG. 15 is a longitudinal sectional view of the ball screw mechanism for use in association with an electrically powered steering device according to a fourth preferred embodiment of the present invention;
FIG. 16A is a longitudinal side view of the ball screw mechanism of FIG. 15 showing the rotary nut employed therein;
FIG. 16B is an end view of the ball screw mechanism shown in FIG. 16A;
FIG. 17A is a fragmentary side view, on an enlarged scale, of the end cap showing a counterbore defined therein;
FIGS. 17B and 17C are fragmentary sectional views showing the counterbore before and after pressing of a projection after a bolt has been screwed in, respectively;
FIG. 18A is an end view of the end cap as viewed internally;
FIG. 18B is a side sectional view, on an enlarged scale, of the end cap showing a reversing passage defined therein;
FIG. 18C is an end view of the rotary nut showing the reversing passage defined therein in communication with a throughhole;
FIG. 18D is a side sectional view, on an enlarged scale, of the rotary nut showing communication between the throughhole and the reversing passage defined in the rotary nut;
FIG. 19A is a schematic sectional view showing a mold used in an injection molding process to form the rotary nut;
FIG. 19B is a schematic sectional view showing a mold used in an injection molding process to form the end cap;
FIG. 20 is a longitudinal side view of the ball screw mechanism, with a portion broken away, for use in association with an electrically powered steering device according to a fifth preferred embodiment of the present invention;
FIG. 21 is a sectional view of the rotary nut used in the ball screw mechanism shown in FIG. 20;
FIG. 22 is an exploded perspective view of the ball screw mechanism shown in FIG. 20;
FIGS. 23A to 23C are schematic longitudinal sectional views showing different prior art ball screw mechanisms; and
FIG. 24 is a fragmentary longitudinal side view of the prior art ball screw mechanism.